They certainly didn't rewrite the framework in one go. WinRT is in its infancy compared to Win32. They have a long way to go.

Correct, but the reason I do not consider WinRT a (full) rewrite is because the most important earlier Win32 APIs are still available. This means porting can be done gradually where newer code can use the new better API, while older code can coexist with it.

That WinRT isn't supported in any form (i.e. not even the System namespace in C++/CX) on Vista and Windows 7 somewhat ruins the possibility to take advantage of their new APIs until Windows 8+ becomes the dominant version of Windows.

I agree that it is a problem for very large apps, but I don't think Microsoft expects those to be ported over particularly

That unfortuantely becomes a problem for Modern because apps from the "hwnd" world does not properly coexist with Modern desktop apps. The poor user experience switching between them is probably Windows 8's biggest problem.

Actually, Windows 8.1 allows a larger variety of snap states (especially on large, high resolution monitors where I think you can snap 4 or more apps at once). There is definitely some better multitasking support there.

What I do not see them address is the key problem: that if a user has 50% win32 apps, and 50% modern apps, then the user experience will be very poor. And it will stay this way for a decade unless they find a proper way to address it. Carbon to Cocoa took this long with slackers like Adobe.